The present invention is concerned with an offset-fed dual-reflector antenna whose main reflector and subreflector are shaped in a non-quadratic surface.
An offset-fed dual-reflector antenna has the feature that its primary radiator and sub-reflector do not cover the aperture of its main reflector, therefore, it gives no unnecessary electromagnetic wave scattering and has an excellent wide angle radiation directivity. By reason of the above fact, it has been in practical use for the communications field and in radar applications.
A conventional Cassegrain antenna of the axial symmetry type which does not offset its sub-reflector has the advantage of obtaining an ideal directivity by means of modifying the electric field distribution at the aperture to a desired one with shaped non-quadratic surfaces of reflectors. On the other hand, an offset-fed dual-reflector antenna has no design freedom to choose a desired electric field distribution at the aperture and this is considered a great drawback to an offset-fed dual reflector antenna. This is due to the following reasons.
When the reflector system of an offset-fed dual-reflector antenna is determined by numerical calculation in general, the following three conditions must be satisfied.
(1) The optical path length from a primary radiator's phase center to an aperture plane is constant for every optical path.
(2) The reflection law (incidence angle of input beam is equal to that of output beam) is satisfied at a sub-reflector.
(3) The reflection law is satisfied at a main reflector.
And, in addition to the above three conditions, the following conditions are necessary for obtaining a desired electric field distribution in the radial direction of an aperture.
(4) An energy distribution condition in radial direction (field distribution on an aperture plane).
Further, for an excellent cross polarization characteristic, the following condition is requested.
(5) The electric field distribution at an aperture in the circumferential direction is axis symmetrical.
A solution satisfying the above five conditions simultaneously is, however, impossible because no solution exists and this is the main reason for said drawbacks.
For example, a certain kind of offset-fed dual-reflector antenna (Japanese Patent application. No. 34652/76 "Antenna of an offset aperture type") has a reflector system satisfying the conditions (1), (2), and (3), and the electric field distribution at an aperture is of axial symmetry because of introducing the condition (5) to suppress the generation of cross polarization components. As a result, the electric field distribution in the radial direction is solely determined because the reflector system is determined completely by the four conditions and there is no room for applying the condition. 4), and a desired field distribution on an aperture plane can not be implemented. Therefore, the directivity of the antenna of this kind cannot be optimized to the surrounding radio circuitry, and the said drawbacks of an offset antenna still remain unsolved in this design method.
Another conventional approximation method has been proposed to provide a desired electric field distribution at least in the vertical plane of a reflector (Japanese utility model application No. 19853/83).
In this method, in the first place, only the vertical central cross section curves of an offset-fed dual-reflector antenna are obtained under said conditions (1), (2), (3), and (4). Then, assuming that the surface of a sub-reflector and a main reflector is comprised of a group of ellipses whose long axis exists on the plane obtained by connecting two points of the corresponding cross section curve. Next, the rest of coordinates other than those of the cross section curve is determined applying the conditions (1) and (2) only. Further, an approximation for the condition (5) is obtained by setting the angle between the primary radiator and the sub-reflector properly.
Accordingly, in this method, a desired electric field distribution is established only in the portion of the vertical central cross section curve and its vicinity, and in other portions of a reflector surface the condition (4) is not satisfied.
Generally, an antenna for use in a microwave relay circuit is expected to have an excellent wide angle radiation directivity in the horizontal plane. As the electric field distribution in the horizontal direction is directly related to the directivity, this design method which does not give a desired electric field distribution on an aperture in the horizontal direction is not suitable for antennas of that purpose.
Considering the antenna design methods stated above, a new design method has been proposed where the central axis of a primary radiator is set parallel to the antenna's main radiation direction (boresight axis), and the reflector surface coordinates are calculated under the said conditions (1), (2), (4), and (5). ( Lee, Parad, Chu, "A Shaped Offset-Fed Dual-Reflector Antenna.", IEEE trans. on AP, AP-27, 2, pp. 165/171, March 1979.)
In this method, however, as the condition (3) is completely ignored and the condition (2) is not considered enough, therefore, an electromagnetic wave reflected by a main reflector and propagates toward the main radiation direction has a variety in the direction of its components. And, as this directional error of each point on an aperture is different in magnitude and direction from one another, the total electromagnetic wave does not converge correctly. In a case where the size of the antenna's aperture is not so large compared with the wave length of an electromagnetic wave, the influence of this effect on the co-polarization characteristic can be neglected. But it brings a serious deterioration of the cross polarization characteristic because of the antenna's design based on the condition (4). And, when the aperture's size is longer than 100 times the wave length, the influence of this effect on the co-polarization characteristic cannot be ignored any longer.